1. Field of the Invention
This invention relates to voltage converters as described in the conference report of the International Power Electronic Conference of Mar. 27-31, 1983 in Tokyo, Japan, pages 1587-1598.
2. Discussion of the Background
In the above noted report, a three-phase inverter bridge circuit, the input side of which is supplied with a direct voltage of 1.5 kV by means of current collectors from a contact wire line, is specified for driving induction motors of an electrically operated vehicle. Each bridge branch has two series-connected gate turn-off thyristors, each being designed for a reverse voltage of 2.5 kV and a current of 1.6 kA. Each thyrisor is associated with a turn-off release circuit which is provided with a circuit diode, in series with a circuit capacitor in a parallel circuit to the thyristor. In parallel with this circuit diode, which has the same polarity as the thryistor, a circuit resistor is connected in series with a winding of a feedback transformer. The energy stored in the two circuit capacitors of the two thyristors can be fed back into the direct-voltage system by means of the feedback transformer. In series with the two thyristors, a simple reactor is provided for limiting the current rise, and a saturable reactor, connected in series with it, for reducing instantaneous overvoltages during the switching on of the thyristors. To recover the energy stored in the two reactors, the winding of a second feedback transformer is connected in series with a diode in a parallel branch to the reactors (free-wheeling branch).
The disadvantageous feature of this is that two relatively expensive gate turn-off thyristors must be connected in series for each inverter branch for a direct feed voltage which is low in comparison with the reverse voltage of the thyristors.